Has anyone ever considered this issue with magnet falling in copper tube?

[tamtam402]

Has anyone ever considered this issue with magnet falling in copper tube??

Homework Statement

I must explain why a magnet falling in a copper tube slows down.

Homework Equations

F = B^qv
However, I must *explain* this, I don't have to calculate anything

The Attempt at a Solution

I understand that the magnet can fall with it's south pole or north pole first, and it still slows down. I understand the direction of the current formed by induction in the copper tube. It's easy to see (I attached a drawing) that the north pole of the magnet faces the north pole of the current caused in the copper tube.

What I don't understand is the stuff happening ABOVE the tube. The way I see it, the south pole will get further and further away from a given point of the tube. Moving a south pole away from a conductor is the same thing as moving a north pole closer to a conductor. That means the current above and below will be going in the same direction. That also means the SOUTH pole of the upper magnetic field will face the SOUTH pole of the magnet.

If the bottom north/north poles are facing each other, I understand why the magnet slows down. However, the south/south poles are facing each other once the magnet has passed a given point of the tube. Why is the magnet slowing down then? Shouldn't it get pushed down by the south/south poles as much as it gets slowed down by the bottom north/north ones?

***NOTE*** I've seen a few drawings while browsing the internet with the current above the magnet going in the opposite direction from the bottom one, which means the upper part would have a north/south pole facing, also slowing down the magnet. I don't understand how the current can go in that direction though...??!

 

[tamtam402]

I added some stuff to my drawing, sorry again about my bad english, I swear I understand this stuff more than what my text shows (it's hard to explain this stuff in an unfamiliar language).

 

[tamtam402]

Oh wow I think I figured it out. I'm going to explain the tendency of a conductor to oppose the variation of flux it "sees". For the bottom part: As the magnet moves closer and closer, the electrons see more and more magnetic field lines "crossing" the circular area of a given disc (with a -y direction on my drawing), explaining why the induced current moves in such a way to create a magnetic field on the positive y side (to balance things out). That means I was correct here.

However, for the upper part: Right after the magnet passes a point, there's a lot of magnetic field lines crossing the circular area in a -y direction. As the magnet keep falling down, less and less magnetic lines cross that same area, explaining why the current moves in a way to "create" more -y direction magnetic field lines, since it wants to oppose the change in magnetic field flux. That means the upper current will be going in the opposing direction, which means there will be a North poles facing the south pole of the magnet.

Is that it?

 

[vela]

Yup, you got it.